A radio frequency (RF) isolator is one example of an RF circuit having a non-reciprocal response. In an ideal RF isolator, RF signals may be allowed to pass in a forward direction and may be completely blocked in a reverse direction. However, practical RF isolators have an insertion loss in the forward direction and a return loss in the reverse direction, which may have a non-uniform frequency response. The RF isolator may be used between a power amplifier and a transmitting antenna to pass transmitted signals from the power amplifier and block reflected signals coming back from the antenna due to impedance mismatch issues, such as antenna loading effects. In a portable wireless device, such as a cell phone, a wireless personal digital assistant (PDA), or the like, antenna loading conditions may be unpredictable and subject to frequent changes, which may cause antenna reflections. By isolating the power amplifier from the antenna reflections, output power stability from the power amplifier may be improved.
An RF isolator that is based on a gyrator, such as one of a Murata CES30 Series, may operate as a bandpass filter in the forward direction and as a single-notch filter in the reverse direction. The single-notch filter has a notch frequency at which the notch filter provides its maximum isolation. As long as the power amplifier is transmitting at or near the notch frequency, the RF isolator may provide adequate isolation from reflected signals. However, some portable wireless devices may be multi-mode devices, which may operate using two or more RF communications bands with wide frequency separation from one another. The RF isolator may provide inadequate isolation for such devices. An RF isolator based on a gyrator, such as another of the Murata CES30 Series, may operate as a dual-notch filter in the reverse direction. The dual-notch filter has a first notch frequency and a second notch frequency. A reverse isolation band spans the frequencies between the first and second notch frequencies, and the reverse isolation band may span two or more RF communications bands. However, the isolation provided by a dual-notch RF isolator in its reverse isolation band may be significantly less than the isolation provided by a single-notch RF isolator at its notch frequency. The isolation provided by the dual-notch RF isolator in its reverse isolation band may be inadequate. Thus, there is a need for an RF isolator that can provide reverse isolation over a wide frequency range with isolation that is equivalent to a single-notch RF isolator at its notch frequency.